Optical transmission systems use optical modulators. Examples of known optical modulators include phase-modulation optical modulators using electro-optic crystals such as LiNbO3 (lithium niobate: LN) or LiTaO2 (lithium tantalite). When such an optical modulator is modularized and installed in an optical transmitter, an optical device such as an optical module, for example, is mounted to the printed circuit board (PCB) of the optical transmitter using surface mounting technology. When the optical device is mounted to the PCB using surface mounting technology, an interconnecting circuit board such as a flexible printed circuit (FPC) is used to electrically connect the optical device to the PCB.
Referring to FIGS. 17 to 19, surface mounting of an optical module will be described. FIG. 17 is a cross-sectional view of connection sections of a PCB and an FPC viewed from a side, FIG. 18 is a diagram illustrating the connection surface of the PCB to be coupled to the FPC, and FIG. 19 is a diagram illustrating the connection surface of the FPC to be coupled to the PCB. For convenience of explanation, the connection surface of the FPC 300 to be coupled to the PCB 400 is referred to as the “bottom surface” and the opposite surface is referred to as the “top surface”. The connection surface of the PCB 400 to be coupled to the FPC 300 is referred to as the “top surface” and the opposite surface is referred to as the “bottom surface”.
As illustrated in FIG. 17, electrodes of the FPC 300 and electrodes of the PCB 400 are conductively interconnected through solder 500. On the connection surface of the PCB 400, CoPlanar Waveguides (CPW) 410 including electrode pads for ground wiring lines 400G1 and 400G2 and an electrode pad for a signal wiring line 400S are formed as illustrated in FIG. 18. Similarly, coplanar waveguides CPW 310 including electrode pads for ground wiring lines 300G1 and 300G2 and an electrode pad for a signal wiring line 300S are formed on the connection surface of the FPC 300 as illustrated in FIG. 19. The signal wiring line of the FPC 300 changes its structure at the end of the CPW to a MicroStrip Line (MSL) 320. The CPW 410 of the PCB 400 and the CPW 310 of the FPC 300 are coupled together by soldering.
The width of the strip of the signal wiring line is designed so that the impedance of the electrodes of the MSL 320, among the electrodes formed on the connection surface of the FPC 300, is equal to a design value, for example 50Ω. The electrodes of the CPW 310 of the FPC 300 are formed as electrode pads in order to widen the area of contact of the signal wiring line 300S, the ground wiring lines 300G1 and 300G2 with solder. Through-holes for checking the quality of solder joint (wetness) are provided in the electrode pads of the signal wiring line 300S and the ground wiring lines 300G1 and 300G2. Solder, if present between a through-hole and the PCB, flows into the through-hole due to capillary action. The quality of the solder joint may be inspected by visually checking the through-holes formed in the top surface of the FPC 300 opposite from the bottom surface, which is the connection surface.
When the FPC 300 is soldered to the PCB 400, a solder bridge may be formed between the signal wiring line 300S and the ground wiring line 300G1 or 300G2 to cause the so-called short circuit if the spacing between the electrode pad of the signal wiring line 300S and the electrode pad of the ground wiring line 300G1 or 300G2 is too small. Therefore, in the CPW 310 of the FPC 300, the electrode pads are disposed at such a distance s1 apart from one another that the possibility of formation of a solder bridge is reduced.
While one end of the FPC that is coupled to the PCB is illustrated in FIGS. 17 to 19, the other end of the FPC is coupled to the tips of lead pins attached through the external wall of a package of the optical module. The lead pins may be attached to a sidewall of the package or near the bottom of the package.
Unfortunately, the conventional technique described above has a problem that electrical signal reflection characteristics at the electrical connections between the printed circuit board and the interconnecting circuit board degrade as described below.    [Patent document] Japanese Laid-Open Patent Publication No. 2004-320109
The interconnecting circuit board described above is intended to be used merely for surface-mounting an optical module for low-frequency application. A wide spacing is provided between electrode pads of a signal wiring line S and a ground wiring line G because a higher priority is given to the ease of surface mounting of the optical module than electrical signal reflection characteristics. The wide spacing between the electrode pads of the signal wiring line S and the ground wiring line G reduces the possibility of occurrence of a solder bridge but increases the impedance to a value higher than a design value. As a result, in an optical module for high-frequency application, impedance mismatches may occur and the electrical signal reflection characteristics at the electrical connection between the printed circuit board and the interconnecting circuit board degrade.